The purpose of this proposal is to develop a novel instrument for mapping the distribution of targeted compounds in tissue sections with a spatial resolution of 20-40 um using atmospheric pressure (AP) matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS). Compared to vacuum MALDI the AP MALDI approach for tissue imaging has such important advantages as easy handling of tissue samples at atmospheric conditions and a tandem MS capability when the AP MALDI source is interfaced to an ion trap mass spectrometer. The latter will provide capability of structural elucidation for molecules under study and bring better specificity via adding 1 more MS dimension into the analysis. The most important feature of this proposal is to use an infrared (IR) laser with the 3-um wavelength for ion desorption instead of an ultraviolet laser. AP IR-MALDI has the potential to operate without adding any matrix onto the sample, using water molecules normally present in the tissue sample as a matrix, thus, substantially simplifying the analysis protocol and allowing direct monitoring of targeted compounds in the tissues without matrix interferences. The proposed technology has a potential of providing important molecular information in many areas of biological research and clinical studies.